Traveling wave tube



1952 R. w. LANDAUER TRAVELING WAVE TUBE Filed Jan. 26, 1949 qU N/ IV/HQ aswad IN V EN TOR. flolfiitljandauer' ATTORNEY Patented Feb. 5, 1952 TRAVELING WAVE TUBE Rolf W. Landauer, New York, N. Y., assignor to Syivania Electric Products Inc., a corporation of Massachusetts Application January 26, 1949, Serial No. 72,912

8 Claims.

The present invention relates to electronic amplifiers of the electron-beam traveling-wave type.

Traveling-wave tubes have acquired an importance in radio circuits that is perhaps unique among high frequency amplifiers because of their capacity to amplify a broad band of frequencies. The problem of separation of the signal from tube-noise is especially difficult for broad-band amplification of ignals of low input power. An object of the present invention is to provide certain improvements in traveling-wave tubes for improved noise figure in amplifying low-level signals.

Traveling-wave tubes are generally considered to include a cathode for supplying electrons, gun electrodes for focusing the electrons and projecting them as a beam down on the tube axis, and a collector electrode, all enclosed in an evacuated envelope. Additionally a wave-guide is arranged about the path of the beam, that is capable of supporting slow waves, the phase velocity of these waves being of nearly the same magnitude as the velocity of the electrons in the beam. Most commonly, the wave-guide is in the form of a helical wire. Coupling means is provided at the extremities of the wave-guide for introducing low-level signals and for delivering amplified signal power. The signal at the input end of the tube is present wholly in the electromagnetic field that is impressed on the waveguide, while at the same input end the electron beam is not modulated except for noise. Interaction between the beam and the electromagnetic ,yfield produces a modulation on the beam that in turn imparts amplified power to the electromagnetic field. The phase of the electromagnetic wave and the electrons in the beam travel down the tube at almost the same speed.

In low-power traveling-wave tubes, noise originates primarily in the cathode, because of thermal agitation. This noise emerges with the beam at the signal-input coupling, and as the beam travels down the tube, the noise is impressed on the waveguide due to the interaction between the beam and the waveguide fields; and the noise, in turn, is amplified together with the signal. The degree of interaction is known to affect the noise figure. Low interaction reduces the noise figure, but also reduces the gain per unit-length of tube. Increasing the tube length to achieve desirable high gain tends to make the tube unduly long, and reduces the band-width that is of such importance in travelins-wave tube applications.

In practicing the present invention, the advantage of low noise figure of low-gain tubes is combined with high gain and broad-band characteristics of tubes of moderate length. The interaction is made low at the input end of the tube and, in a following part of the tube, where noise is present in significant measure in the electromagnetic field and the signal is similarly present as densityor velocity-modulation, in the beam, the interaction is increased. This is very different from arranging plural traveling-wave tubes in cascade, the first of low gain, low interaction characteristics and the other or others of high gain-per-unit length and close coupling characteristics. In such a hypothetical arrangement the signal output of the first tube tends to be of such low level that the noise of the following high-gain tube is added to that of the first tube. In providing a single tube of reduced interaction at the input end and increased interaction further down the tube, the signal is present as beam modulation in the region that.

the high-gain section of tube commences and is not required as in the hypothetical case to impress modulation on the beam.

In principle, the interaction or coupling can be increased gradually or stepwise along the tube. However it has been found that a satisfactory, practical approach is to construct the tube in two sections, first of a low-coupling factor and then of high-coupling factor, between input and output ends of the tube, respectively.

The nature of the invention, and its further features and purposes will be appreciated from the following detailed disclosure of an illustrative embodiment shown in the accompanying drawings wherein:

Fig. 1 is a longitudinal cross-section of an illustrative .form of traveling-wave tube in ac cordance with the present invention; and

Fig. 2 is a graph representing the relative beam modulation of signal and noise, along travelingwave tubes reduced to a common basis of comparison. This curve is capable of other interpretation and can serve other purposes.

The following description and explanation of the mode of operation of a two-sectional traveling ing-wave tube is on an intuitive basis, rather than on a more rigorous mathematical basis. The general mathematical considerations are treated in a paper by J. R. Pierce entitled "Theory of the Beam Type Traveling Wave Tube, proceedings of the IRE, volume 35, pages 111 through 123, February 1947. The qualitative development can be mathematically demonstrated, upon the eludes an input section of low interaction and an output section of high interaction. In Fig. 1, the electron beam which emerges from gun l0 travels along a path that is straight in this instance, through tubular signal-input coupler I2 and axially down the evacuated tubular envelope H as of quartz to collector l6. Technically element l2, plus the first few turns of the helix act as the input coupler, and the extension of element I2 toward the gun outside the (dotted) input waveguide acts as a choke to prevent radiation. In so doing it travels down the axis of a first wire helix l8, through the central apertures of three discs 20, 22 and 24 that are sealed through the wall of tube [4 to provide external terminals, through a second metal helix 26 and through tubular output coupler 28. The discs are insulated apart so that helices l8 and 26 can be maintained at different potentials. Magnetic focusing coils 30 and 32 are provided for maintaining the beam in focus until it leaves the sec ond helix. A film of vapor-deposited Nichrome is on the inside tube wall on opposite sides of discs 20 and 24 for attenuating the electromagnetic wave. This prevents amplified reflections from the output end of the tube from feeding back to the input end of the tube. Features of this construction are covered in copending application Serial No. 54,676, filed October 15, 1948, by Francis C. Breeden and Leo C. Eisaman.

Considering the electrostatic aspect of the beam travel, the electrons emerging from gun I0 are accelerated by probe l2 and helix H! by connection to a point of higher electrostatic potential than the gun, although probe l2 and helix I 8 may be maintained at the same potential as the final anode of gun Hi. In any event, apertured discs 20, 22 and 24 are desirably connected to external direct-current supplies (not shown) so as to accelerate the electron beam to higher velocity as it enters the second helix 26. The diameters of helices l8 and 26 differ in this illustrative embodiment, and the number of turns per inch of helix difiers in the two sections.

The important parameter characterizing the different sections is variously called the interaction factor, gain parameter or coupling factor, represented by C. This parameter is an expression that involves electric field strength of the waves, power transmitted along the wave guide, propagation constants of the guide without the electron beam, and current and velocity of the electron beam. The parameter appears in expressions determining the phase velocities of waves in the presence of the electron beam, the gain and noise figure of the device.

In the case of the helical wave guide the expression is given by:

where E2 is the z-component of the electric field on the axis of the guide in the absence of the electron beam.

P0 is the power transmitted along the waveguide in the absence of the electron beam in the zero order mode.

ID is the direct current in the electron beam.

V0 is the voltage measuring the direct-current beam speed.

3 is the phase constant of the wave amplified, and equals 21rf/v where is frequency and v is the phase velocity along the tube. The phase velocity is related in a highly complex manner to the radius of the helix and the number of turns per inch.

(The asterisk indicates the conjugate complex of the quantity.)

The noise figure of a single helix travelingwave tube amplifier is given by:

where 1 is the so-called space-charge smoothing factor.

The interaction or coupling factor depends on the frequency and the beam current, both of which may be considered to be alike for both sections of a two-sectional helix, and it depends also on V0, the radius and the number of turns per inch, which can be made different for each helix to make the coupling factor for the two sections different. Therefore, while the coupling factor generally varies with operating conditions, a traveling-wave tube having different coupling factors involves different physical characteristics at different parts as turns-per-inch and radius of helix, and/or insulation between sections of helix.

In the illustrative tube, the first section of helix is about one-third of the total tube-length occupied by both helices, and has fewer turns per inch and larger diameter. The two sections of helix are insulated apart for maintenance of different voltage in the two helices, and apertured discs 20, 22 and 24 help to maintain beam focus despite the changed beam velocity. The higher-gain section normally will have higher electron-beam velocity. so that the diameter and turns-per-inch are the other physical variables that will produce the desired coupling.

The first section of the tube is, then, to be of low interaction proportions, which section is to extend only partway toward the output end of the tube. In Fig. 2, tube gain is plotted against ON. The coupling factor C has been explained above, and N is a measure of tube length in terms of wavelengths. From this it can be seen that little advantage is to be realized by increasing CN beyond 0.3, insofar as noise figure is concerned. From this point on, along the tube, the noise and signal are affected nearly alike by the electron beam-to-waveguide interaction, and thereafter, a tube section of high gain per unit-length can be used without penalty in respect to noise and can be used to advantage in broadening the bandwidth of the tube and in shortening the over-all physical length of the tube for given conditions of energization and gain.

The foregoing illustrative embodiment of the invention described in detail will be recognized as subject to varied modification by those skilled in the art and for this reason the appended claims should be allowed such broad interpretation as is commensurate with the spirit and scope of the invention.

What is claimed is:

1. An electronic amplifier comprising means for projecting an electron beam along a path, means for guiding the propagation of electromagnetic waves along said path, whereby said beam is modulated in accordance with the ropagated waves, the coupling factor of the amplifier being lower at the input end than at the output end of the electron path.

2. An electronic amplifier having means for projecting an electron beam along a path, means for guiding the propagation of electromagnetic Waves along said path whereby said beam is modulated in accordance with the propagated waves, said guiding means being divided into multiple successive sections, the coupling factor C of the amplifier being lower at the input section than the remainder of the guiding means, and the product of the coupling factor C and the electrical length N of the input section of wave guiding means being approximately 0.3.

3, An electronic amplifier comprising means for projecting an electron beam along a path, means for guiding the propagation of electromagnetic waves along said path whereby said beam is modulated in accordance with the propagated wave, said wave guiding means being divided into multiple sections mutually insulated apart and having separate external terminals, the coupling between the wave guiding means and the beam being lower at the input end than at the output end of the electron path.

4. An electronic amplifier comprising means for projecting an electron beam along a path, means for guiding the propagation of electromagnetic waves along said path whereby said beam is modulated in accordance with the propagated wave, said wave guiding means being divided into multiple sections mutually insulated apart and having separate external terminals, the coupling between the wave guiding means and beam being lower at the input end than at the output end of the electron path, and beam focusing electrodes between adjacent sections of said wave guiding means.

5. An electronic amplifier comprising means for projecting an electron beam along a path, means for guiding the propagation of electromagnetic waves along said path whereby said beam is modulated in accordance with the propagated wave, said wave guiding means being divided into multiple sections mutually insulated apart and having separate external terminals,

the input section of said wave guiding means being a helix of a certain diameter about the electron path, the next section of wave guiding means being a helix about the electron beam path, but of smaller diameter than the input section of wave guiding means.

6. An electronic amplifier comprising means for projecting an electron beam along a path, and plural sections of conductive helix in succession along said electron beam path, said sections being, insulated apart.

7. An electronic amplifier including means for projecting an electron beam along a path, input modulating means closely adjacent to that path for imparting input signal modulation to the beam, output-signal deriving means at the opposite end of that path, plural sections of cc nductive helix located in succession along the electron beam path, said sections of helix being separated by electrical insulation. and separate terminals for energizing said sections at different beamaccelerating direct-current potentials.

8. An electron amplifier including means for projecting an electron beam along a path, means for guiding the propagation of electromagnetic waves along said path whereby said beam is modulated in accordance with the propagated Waves, said guiding means having a different relationship to the beam projecting means at the input end thereof than at the output thereof, these relationships affording a lower coupling factor at the input end than at the output end of the beam path.

ROLF W. LANDAUER.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 2,300,052 Lindenblad Oct. 2'7, 1942 OTHER REFERENCES Article: "Wideband Microwave Amp. Tube,

pages -92 of "Electronics for November 1946.

Article: "The Beam Traveling-Wave Tube," pages 439-442, Bell Lab. Record for December 

